[Paper Review] Doping of Mn$_2$VAl and Mn$_2$VSi Heusler alloys as a route to half-metallic antiferromagnetism

I. Galanakis, K. Özdoğan+2 more|ArXiv.org|Jan 25, 2007

Heusler alloys: electronic and magnetic properties4 citations

TL;DR

This paper proposes Co-doping of Mn2VAl and Mn2VSi Heusler alloys as a route to achieve half-metallic antiferromagnetism (HMA), a desirable state for spintronic devices due to zero net magnetization and minimal stray fields. Using first-principles calculations, it shows that when the total valence electron count reaches 24, Co-doped compounds become HMA due to strong Mn-Co hybridization that drives magnetic ordering via the Stoner criterion, while Fe-doped analogs become non-magnetic semimetals.

ABSTRACT

Half-metallic antiferromagnets are the ideal materials for spintronic applications since their zero magnetization leads to lower stray fields and thus tiny energy losses. Starting from the Mn$_2$VAl and Mn$_2$VSi alloys we substitute Co or Fe for Mn and we show by means of first-principle electronic structure calculations that the resulting compounds are ferrimagnets. When the total number of valence electrons reaches the magic number of 24 the Fe-doped compounds are semi-metals and thus non-magnetic while the Co-doped ones show the desirable half-metallic antiferromagnetic character. The compounds are very likely to be synthesized experimentally since the parent compounds, Mn$_2$VAl and Co$_2$VAl, have been already grown in the Heusler $L2_1$ lattice structure.

Motivation & Objective

• To explore doping strategies for achieving half-metallic antiferromagnetism (HMA) in Heusler alloys, which are ideal for spintronic applications due to zero net magnetization and minimal energy loss.
• To address the challenge of realizing HMA in real materials, as ideal HMA compounds like MnCrSb or Mn3Ga do not crystallize in stable structures.
• To investigate whether Co or Fe substitution for Mn in Mn2VAl and Mn2VSi can stabilize HMA while preserving the half-metallic character.
• To determine the electronic and magnetic conditions under which HMA emerges, particularly the role of valence electron count and hybridization effects.

Proposed method

• First-principles electronic structure calculations using the full-potential linearized augmented plane wave (FPLO) method with the local density approximation (LDA).
• Application of the coherent potential approximation (CPA) to model random substitutional doping of Mn with Co or Fe.
• Calculation of total and atom-resolved density of states (DOS) to analyze electronic structure and identify gaps at the Fermi level.
• Use of the Slater-Pauling rule to predict total spin moments based on valence electron count, with 24 electrons as the critical threshold.
• Analysis of the Stoner criterion to determine the stability of magnetic vs. non-magnetic states in the non-magnetic limit.
• Comparison of Fe-doped and Co-doped systems to identify the role of Mn-Co hybridization in driving magnetic ordering.

Experimental results

Research questions

• RQ1 Can Co-doping of Mn2VAl and Mn2VSi transform these half-metallic ferrimagnets into half-metallic antiferromagnets?
• RQ2 What is the role of the 24-valence-electron condition in determining the magnetic ground state of doped Heusler compounds?
• RQ3 Why do Co-doped systems form HMA while Fe-doped systems become non-magnetic semimetals despite both reaching 24 valence electrons?
• RQ4 How does the hybridization between Mn and dopant (Co or Fe) orbitals influence the density of states at the Fermi level and the stability of magnetic order?
• RQ5 Can the Stoner criterion explain the preference for magnetic ordering in Co-doped systems but not in Fe-doped systems when both are non-magnetic at 24 valence electrons?

Key findings

• Co-doping Mn2VAl and Mn2VSi at concentrations where the total valence electron count reaches 24 results in half-metallic antiferromagnetic ground states, with a gap in the spin-up channel and zero net spin moment.
• The HMA state arises because the non-magnetic phase of Co-doped compounds exhibits a high density of states at the Fermi level due to strong Mn-Co hybridization, satisfying the Stoner criterion and favoring magnetic ordering.
• In contrast, Fe-doped compounds with 24 valence electrons become non-magnetic semimetals, as the Fermi level lies within a pseudogap and the Stoner criterion is not met.
• The atomic spin moments in Co-doped systems show antiparallel alignment between Mn and Co, consistent with ferrimagnetic order, with total spin moments decreasing from -2.0 μB at x=0 to near zero at x=1.
• The V atoms maintain a consistent moment of ~0.9–1.1 μB across doping levels, while Mn moments remain around -1.5 μB, indicating the stability of the ferrimagnetic configuration.
• The predicted HMA behavior in Co-doped Mn2VAl and Mn2VSi is experimentally feasible, as both Mn2VAl and Co2VAl have been synthesized in the Heusler L21 structure.